best way to control rotary table with stamp???

Hello, i'm looking for a little bit of help with driving a rotary table with a basic stamp. I am looking for the simplist mechanical
device but yet keeping a resolution of around .1 degrees. I was thinking of using a rc servo driven by a pwm signal and count pulses on an attached encoder. From there drive a 10:1 reduction. I am thinking that it won't work because of the single string design of the stamp. I would test it but i fried my old stamp and a new one is in the mail. I have considered using a stepper but i was trying to limit the number of pins used on the stamp as well as all of the steppers i have are 15 degrees and i would prefer to not have such a large reduction due to space constraints and backlash issues. any suggestions are welcome just trying to find the best way to do it. Thanks in advance
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On May 17, 3:31 pm, snipped-for-privacy@gmail.com wrote:

I would say a servo would work fine. But how many degrees do you need the table to turn? If you have a regular rc servo that doesn't quite turn 360 degrees and then gear it down 10:1, you would get even less rotation wouldn't you? And I'm not sure about the single string design of the stamp. In my experience it has been very easy to drive rc servos with the stamp. If you were to get a digital rc servo you would only have to give it one pulse to tell it want position you want it to go to. Saving some processing power.
Joe McKibben
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Hi,
Why not use a stepper motor? Hardware wise it's a normal motor and from the CPU you just need 2 pins if you use a L297
Regards Ian Dobson
Home of the Atmel based UDP mobile web cam http://www.planet-ian.com All mails scanned with av-filter.pl (F-Prot / perl)

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I assume you are using a R/C Servo with the idea of modifying it for continuous rotation. So, that gives you direction and speed control of the motor.
Your encoder will need to have two channels, to allow for you to detect pulses, and direction. You will need two pins for that, and will also need to ensure that your stamp has enough free time to process the pulses without missing any.
I wonder if there might be a dedicated stepper/encoder setup, which has a serial control line, and is price friendly for the hobbiest? I just googled, and came across this in the UK, http://www.active-robots.com/products/motorcon/motion-mind.shtml
Anyone else have a source for something like this?
Joe Dunfee
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snipped-for-privacy@gmail.com wrote:

Make one of these:
http://www.bmumford.com/rotary/index.html
It is not too hard but you will obviously need to find and mount a stepper motor an buld a suitable driver. The results are excellent however. A stepper will be much easier to couple than a servo and there are no end stops.
A rotary table might have a 40:1 reduction in the worm drive and the stepper has 400 steps per revolution giving you 360/16000 = 0.0225 deg resolution.
Here is another one: http://www.embeddedtronics.com/rotary.html
Pete
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snipped-for-privacy@gmail.com wrote:

From a hardware standpoint I'm with Pete: use a stepper. Even with full step, and the typical 1.8 degree step, you'll have 0.18 degrees with a 10:1 reduction. If you half step, which is easy in software, you'll get about 0.09 degrees. BUT...the half-steps usually aren't equidistant. If you want more accurate positioning either get a higher resolution stepper, or go with a higher gear reducer. You can limit backlash by going with a worm drive.
Those 15 degree steppers are probably pretty old or were pull-outs from old equipment, but in any case, it's easy enough to find 1.8 degree steppers surplus. Try Herbach & Rademan, C&H, the usual places.

I'd use a stepper controller chip, such as the L297, coupled to an L298 (use www.findchips.com to locate sellers of these). You control the L297 with step and direction pulses; it provides the translation. The L298 is a power driver. The L297 is quite old but available lots of places; you can find translator/driver chips from Allegro. The more modern chips have at least full and half step modes, and many have wave and microstep modes. Most are under $10, and you can sometimes get free samples. They're not always available in DIP packaging, though.
In any case, the BASIC Stamp is perfectly suited for this task. You can rig up an optical encoder easily as well. I'd go with the VLSI decoder chips from US Digital -- about $3.50 each. They connect to any quatrature encoder, and provide step and direction pulses. Makes it very, very easy to interface to a Stamp.
-- Gordon
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